Firebase function to add document with data to Firestore only works second and subsequent times of executing it, but not first

Question

I'm getting a little problem which I'm not being capable to debug. I wrote a little Firebase Function to get data from a JSON object and to store it in a Firestore Document. Simple.

It works, except the first time I run it after deployed (or after a long time has passed since the last execution). I have to run it once (without working), and then the subsequent tries always work, and I can see the new document being created with all the data inside it.

In the first attempt, there are no logs: Function execution took 601 ms, finished with status code: 200. Despite that, no document is being created nor changes being made.

In the second and subsequent attempts, If I request the function execution with a HTTP POST to https://cloudfunctions/functionName?id=12345, then the document '12345' is created inside collection with all the data inside it.

The collection where the documents are stored (scenarios) already exist in the database before any function call is executed.

This is the code:

const functions = require("firebase-functions");
const admin = require("firebase-admin");
admin.initializeApp();
const db = admin.firestore();
db.settings({ignoreUndefinedProperties: true});
const fetch = require("node-fetch");
let scenarioData;
const fetchScenarioJSON = async (scenarioId) => {
  try {
    const response = await fetch(`https://url/api/scenarios/single/${scenarioId}`);
    const scenarioText = await response.text();
    scenarioData = JSON.parse(scenarioText);
  } catch (err) {
    return ("not valid json");
  }
  return scenarioData;
};

/**
 * Add data to Firestore.
 * @param {JSON} scenario JSON array containing the scenario data.
 */
async function addDataToFirestore(scenario) {
  const data = {
    id: scenario.scenario._id,
    name: scenario.scenario.name,
    description: scenario.scenario.description,
    language: scenario.scenario.language,
    author: scenario.scenario.author,
    draft: scenario.scenario.draft,
    last_modified: scenario.scenario.last_modified,
    __v: scenario.scenario.__v,
    duration: scenario.scenario.duration,
    grade: scenario.scenario.grade,
    deleted: scenario.scenario.deleted,
    view_count: scenario.scenario.view_count,
    comments_count: scenario.scenario.comments_count,
    favorites_count: scenario.scenario.favorites_count,
    activities_duration: scenario.scenario.activities_duration,
    activities: scenario.scenario.activities,
    outcomes: scenario.scenario.outcomes,
    tags: scenario.scenario.tags,
    students: scenario.scenario.students,
    created: scenario.scenario.created,
    subjects: scenario.scenario.subjects,
  };
  const res = await db.collection("scenarios").doc(scenario.scenario._id).set(data);
}

exports.functionName =
functions.https.onRequest((request, response) => {
  return fetchScenarioJSON(request.query.id).then((scenario) => {
    if (typeof scenario === "string") {
      if (scenario.includes("not valid json")) {
        response.send("not valid json");
      }
    } else {
      addDataToFirestore(scenario);
      response.send(`Done! Added scenario with ID ${request.query.id} to the app database.`);
    }
  });
});

My question is if I am doing anything wrong with the code that makes the execution not work on the first call after it is deployed, but actually does work in subsequent calls.

Answer 1

It is most probably because you don't wait that the asynchronous addDataToFirestore() function is completed before sending back the response.

By doing

  addDataToFirestore(scenario);
  response.send()

you actually indicate (with response.send()) to the Cloud Function platform that it can terminate and clean up the Cloud Function (see the doc for more details). Since you don't wait for the asynchronous addDataToFirestore() function to complete, the doc is not written to Firestore.

The "erratic" behaviour (sometimes it works, sometimes not) can be explained as follows:

• In some cases, your Cloud Function is terminated before the write to Firestore is fully executed, as explained above.
• But, in some other cases, it may be possible that the Cloud Functions platform does not immediately terminate your CF, giving enough time for the write to Firestore to be fully executed. This is most probably what happens after the first call: the instance of the Cloud Function is still running and then the docs are written with the "subsequent calls".

---

The following modifications should do the trick (untested). I've refactored the Cloud Function with async/await, since you use it in the other functions.

// ....

async function addDataToFirestore(scenario) {
    const data = {
        id: scenario.scenario._id,
        name: scenario.scenario.name,
        description: scenario.scenario.description,
        language: scenario.scenario.language,
        author: scenario.scenario.author,
        draft: scenario.scenario.draft,
        last_modified: scenario.scenario.last_modified,
        __v: scenario.scenario.__v,
        duration: scenario.scenario.duration,
        grade: scenario.scenario.grade,
        deleted: scenario.scenario.deleted,
        view_count: scenario.scenario.view_count,
        comments_count: scenario.scenario.comments_count,
        favorites_count: scenario.scenario.favorites_count,
        activities_duration: scenario.scenario.activities_duration,
        activities: scenario.scenario.activities,
        outcomes: scenario.scenario.outcomes,
        tags: scenario.scenario.tags,
        students: scenario.scenario.students,
        created: scenario.scenario.created,
        subjects: scenario.scenario.subjects,
    };
    await db.collection("scenarios").doc(scenario.scenario._id).set(data);
}

exports.functionName =
    functions.https.onRequest(async (request, response) => {
        
        try {
        
            const scenario = await fetchScenarioJSON(request.query.id);
            if (typeof scenario === "string") {
                if (scenario.includes("not valid json")) {
                    response.send("not valid json");
                }
            } else {
                await addDataToFirestore(scenario);  // See the await here
                response.send(`Done! Added scenario with ID ${request.query.id} to the app database.`);
            }
        } catch (error) {
            // ...
        }

    });